Ext.define('IM.model.Category', {
    extend: 'Ext.data.Model',
    fields: [{
    	name: 'compositeId', type: 'string', 
         	convert: function(value, record) {
         		return record.get('productId') + ':' +
         			record.get('id');
        		
         	}
         
         },
        {name: 'id', type: 'int'},
        'name',
        {name: 'productId', type: 'int'},
     ],
     idProperty: 'compositeId'
});